Two hundred years ago, a series of powerful temblors devastated what is now Missouri. Could it happen again?

Over the past three years, Magnani has used these tools to map the ground below the river in the New Madrid Seismic Zone, a fault system stretching about 150 miles from Cairo, Illinois, to Marked Tree, Arkansas. It’s the most active seismic area in the United States east of the Rockies, with about 200 small quakes a year.

Magnani’s most startling discovery came south of the seismic zone: two faults, one near Memphis, both active in the past 10,000 years. Other researchers have recently identified faults near Commerce, Missouri, and other places outside the New Madrid Seismic Zone that have been active in the past few thousand to millions of years, suggesting that the middle of the country is less stable than it seems.

Geologists have long blamed the New Madrid earthquakes on the Reelfoot Rift, a 500-million-year-old area of weakness in the crust. But the newfound faults lie outside the rift. “Maybe the reason we haven’t been able to solve the mystery of the New Madrid earthquakes is that we’ve been too focused on New Madrid,” Magnani says. “Maybe earthquake activity moves around systematically over time.”

Tuttle has begun a four-year project to date sand blows inside and outside the New Madrid Seismic Zone. “We’ve got to get a solid understanding of what large earthquakes happened where and when,” she says. That’s the best way to estimate the hazard to the Midwest and its millions of people and countless highways, bridges, skyscrapers and crumble-prone brick buildings.

The U.S. Geological Survey estimates that the risk of another New Madrid-scale catastrophe in the next 50 years is about 7 to 10 percent. The risk of a smaller, though still devastating, 6.0 earthquake in the next 50 years is 25 to 40 percent. Ongoing research should help identify which areas are most in danger.

“We need a broader, yet clearer picture of all the networks of faults that have been active in the region,” says Magnani. “We need to find out how big they are and their underlying structures. That’s the only way we can hope to understand intraplate earthquakes—and ultimately keep people safe.”

Elizabeth Rusch wrote about extracting energy from ocean waves for Smithsonian.